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  Optional
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  <p>背景: Java 中最常见的异常就是 <code>java.lang.NullPointerException.</code></p>
<p>Optional 类应运而生, <strong>避免代码抛出 NPE 异常</strong></p>
<h3 id="Optional用处"><a href="#Optional用处" class="headerlink" title="Optional用处"></a><strong>Optional用处</strong></h3><ol>
<li>本质上，Optional是一个包含有可选值的包装类，这意味着 Optional 类既可以含有对象也可以为空。</li>
<li>我们要知道，Optional 是 Java 实现函数式编程的强劲一步，并且帮助在范式中实现。但是 Optional 的意义显然不止于此。</li>
<li>任何访问对象方法或属性的调用都可能导致 <code>NullPointerException</code>，在这里，我举个简单的例子来说明一下<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">String result = test.getName().getTime().getNum().getAnswer();</span><br></pre></td></tr></table></figure>
1. 在上面的这个代码中，如果我们需要确保不触发异常，就得在访问每一个值之前对其进行明确地检查，就是使用if else对test等值进行判断是否为null，这很容易就变得冗长，难以维护。<blockquote>
<p> 为了简化这个过程，Google公司著名的Guava项目引入了Optional类，Guava通过使用检查空值的方式来防止代码污染，并鼓励程序员写更干净的代码。</p>
</blockquote>
</li>
</ol>
<p>2. Optional实际上是个容器：它可以保存类型T的值，或者仅仅保存null。Optional提供很多有用的方法，这样我们就不用显式进行空值检测。</p>
<h3 id="一、Optional的构造函数"><a href="#一、Optional的构造函数" class="headerlink" title="一、Optional的构造函数"></a><strong>一、Optional的构造函数</strong></h3><p>Optional 的三种构造方式：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">Optional.of(obj)，</span><br><span class="line">Optional.ofNullable(obj) ,</span><br><span class="line">明确的 Optional.empty()</span><br></pre></td></tr></table></figure>
<p>**Optional.of(obj)**：<br>它要求传入的 obj 不能是 null 值的, 否则直接报NullPointerException 异常。</p>
<p><strong>Optional.ofNullable(obj)：</strong><br>它以一种智能的，宽容的方式来构造一个 Optional 实例。来者不拒，传 null 进到就得到 Optional.empty()，非 null 就调用 Optional.of(obj).</p>
<p><strong>Optional.empty()：</strong><br>返回一个空的 Optional 对象</p>
<h3 id="二、Optional的常用函数"><a href="#二、Optional的常用函数" class="headerlink" title="二、Optional的常用函数"></a><strong>二、Optional的常用函数</strong></h3><p><strong>of</strong>：<br>为非null的值创建一个Optional。of方法通过工厂方法创建Optional类。需要注意的是，创建对象时传入的参数不能为null。如果传入参数为null，则抛出NullPointerException。因此不经常用。</p>
<p><strong>ofNullable</strong>：<br>为指定的值创建一个Optional，如果指定的值为null，则返回一个空的Optional。</p>
<p><strong>isPresent</strong>：<br>如果值存在返回true，否则返回false。</p>
<p><strong>ifPresent</strong>：<br>如果Optional实例有值则为其调用consumer，否则不做处理</p>
<p><strong>get</strong>：<br>如果Optional有值则将其返回，否则抛出NoSuchElementException。因此也不经常用。</p>
<p><strong>orElse</strong>：<br>如果有值则将其返回，否则返回指定的其它值。</p>
<p><strong>orElseGet</strong>：<br>orElseGet与orElse方法类似，区别在于得到的默认值。orElse方法将传入的字符串作为默认值，orElseGet方法可以接受Supplier接口的实现用来生成默认值</p>
<p><strong>orElseThrow</strong>：<br>如果有值则将其返回，否则抛出supplier接口创建的异常。</p>
<p><strong>filter</strong>：<br>如果有值并且满足断言条件返回包含该值的Optional，否则返回空Optional。</p>
<p><strong>map</strong>：<br>如果有值，则对其执行调用mapping函数得到返回值。如果返回值不为null，则创建包含mapping返回值的Optional作为map方法返回值，否则返回空Optional。</p>
<p><strong>flatMap</strong>：<br>如果有值，为其执行mapping函数返回Optional类型返回值，否则返回空Optional。</p>
<h3 id="三、Optional-应该怎样用"><a href="#三、Optional-应该怎样用" class="headerlink" title="三、Optional 应该怎样用"></a><strong>三、Optional 应该怎样用</strong></h3><ol>
<li>在使用 Optional 的时候需要考虑一些事情，以决定什么时候怎样使用它。重要的一点是 Optional 不是 Serializable。因此，它不应该用作类的字段。</li>
<li>如果你需要序列化的对象包含 Optional 值，Jackson 库支持把 Optional 当作普通对象。也就是说，Jackson 会把空对象看作 null，而有值的对象则把其值看作对应域的值。这个功能在 jackson-modules-java8 项目中。</li>
<li>Optional 主要用作返回类型，在获取到这个类型的实例后，如果它有值，你可以取得这个值，否则可以进行一些替代行为。Optional 类可以将其与流或其它返回 Optional 的方法结合，以构建流畅的API。我们来看一个示例，我们不使用Optional写代码是这样的<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">public String getName(User user)&#123; </span><br><span class="line">    if(user == null)</span><br><span class="line">        return &quot;Unknown&quot;; </span><br><span class="line">    else </span><br><span class="line">        return user.name(); </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></li>
</ol>
<ul>
<li>接着我们来改造一下上面的代码，使用Optional来改造，我们先来举一个Optional滥用，没有达到流畅的链式API，反而复杂的例子，如下<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">public String getName(User user)&#123; </span><br><span class="line">    Optional&lt;User&gt; u = Optional.ofNullable(user); </span><br><span class="line">    if(!u.isPresent())&#123;</span><br><span class="line">        return &quot;Unknown&quot;; </span><br><span class="line">    else </span><br><span class="line">        return u.get().name(); </span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></li>
<li>这样改写非但不简洁，而且其操作还是和第一段代码一样。无非就是用isPresent方法来替代原先user==null。这样的改写并不是Optional正确的用法，我们再来改写一次。<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">public String getName(User user)&#123; </span><br><span class="line">    return Optional.ofNullable(user) .map(u -&gt; u.name) .orElse(&quot;Unknown&quot;); </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></li>
<li>这样才是正确使用Optional的姿势。那么按照这种思路，我们可以安心的进行链式调用，而不是一层层判断了。当然，我们还可以通过getter方式，对代码进行进一步缩减（前提是User要有getter方法哦），如下<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">String result = Optional             </span><br><span class="line">                    .ofNullable(user)</span><br><span class="line">                    .flatMap(User::getAddress)</span><br><span class="line">                    .flatMap(Address::getCountry)</span><br><span class="line">                    .map(Country::getIsocode)</span><br><span class="line">                    .orElse(&quot;default&quot;);</span><br></pre></td></tr></table></figure></li>
</ul>
<hr>
<p>  <code>of(Object value)    </code>       将非空值设置到Optional实例并返回该实例</p>
<p> <code>ofNullable(Object value) </code> 将值设置到Optional实例中并返回该实例</p>
<p>  <code>get()</code>                      非空则返回Optional内存的值，为空报错NoSuchElementException</p>
<p>  <code>empty()</code>                    返回一个空 Optional实例</p>
<p>  <code>orElse(Object other) </code>      如果当前有值且非空则返回该值，否则返回 other</p>
<p>  <code>isPresent()</code>                如果值存在则方法会返回true，否则返回 false。</p>
<hr>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">Optional.of(2); </span><br><span class="line">System.out.println(Optional.of(4)); //输出：Optional[4]</span><br><span class="line"></span><br><span class="line">System.out.println(Optional.of(4).get()); //输出：4</span><br><span class="line"></span><br><span class="line">System.out.println(Optional.ofNullable(null)); //输出：Optional.empty </span><br><span class="line">System.out.println(Optional.ofNullable(null).get());//报错 </span><br><span class="line">System.out.println(Optional.ofNullable(null).orElse(&quot;2222&quot;)); //输出2222 </span><br><span class="line">System.out.println(Optional.ofNullable(&quot;1111&quot;).orElse(&quot;2222&quot;));//输出1111 </span><br><span class="line">System.out.println(Optional.ofNullable(null).isPresent()); //输出false </span><br><span class="line">System.out.println(Optional.ofNullable(&quot;1111&quot;).isPresent()); //输出true</span><br></pre></td></tr></table></figure> 
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    headingSelector: "h1, h2, h3, h4, h5, h6",
    hasInnerContainers: true,
    scrollSmooth: true,
    scrollContainer: "main",
    positionFixedSelector: ".tocbot",
    positionFixedClass: "is-position-fixed",
    fixedSidebarOffset: "auto",
  });
</script>

<script src="https://cdn.jsdelivr.net/npm/jquery-modal@0.9.2/jquery.modal.min.js"></script>
<link
  rel="stylesheet"
  href="https://cdn.jsdelivr.net/npm/jquery-modal@0.9.2/jquery.modal.min.css"
/>
<script src="https://cdn.jsdelivr.net/npm/justifiedGallery@3.7.0/dist/js/jquery.justifiedGallery.min.js"></script>

<script src="/hexo1/dist/main.js"></script>

<!-- ImageViewer -->
 <!-- Root element of PhotoSwipe. Must have class pswp. -->
<div class="pswp" tabindex="-1" role="dialog" aria-hidden="true">

    <!-- Background of PhotoSwipe. 
         It's a separate element as animating opacity is faster than rgba(). -->
    <div class="pswp__bg"></div>

    <!-- Slides wrapper with overflow:hidden. -->
    <div class="pswp__scroll-wrap">

        <!-- Container that holds slides. 
            PhotoSwipe keeps only 3 of them in the DOM to save memory.
            Don't modify these 3 pswp__item elements, data is added later on. -->
        <div class="pswp__container">
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
        </div>

        <!-- Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed. -->
        <div class="pswp__ui pswp__ui--hidden">

            <div class="pswp__top-bar">

                <!--  Controls are self-explanatory. Order can be changed. -->

                <div class="pswp__counter"></div>

                <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>

                <button class="pswp__button pswp__button--share" style="display:none" title="Share"></button>

                <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
                <!-- element will get class pswp__preloader--active when preloader is running -->
                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                        </div>
                    </div>
                </div>
            </div>

            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
            </div>

            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
            </button>

            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
            </button>

            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/default-skin/default-skin.min.css">
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script> 
<!-- MathJax -->

<!-- Katex -->

<!-- busuanzi  -->
 
<script src="/hexo1/js/busuanzi-2.3.pure.min.js"></script>
 
<!-- ClickLove -->

<!-- ClickBoom1 -->

<!-- ClickBoom2 -->

<!-- CodeCopy -->
 
<link rel="stylesheet" href="/hexo1/css/clipboard.css">
 <script src="https://cdn.jsdelivr.net/npm/clipboard@2/dist/clipboard.min.js"></script>
<script>
  function wait(callback, seconds) {
    var timelag = null;
    timelag = window.setTimeout(callback, seconds);
  }
  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>
 
<!-- CanvasBackground -->

<script>
  if (window.mermaid) {
    mermaid.initialize({ theme: "forest" });
  }
</script>


    
    <div id="music">
    
    
    
    <iframe frameborder="no" border="1" marginwidth="0" marginheight="0" width="200" height="52"
        src="//music.163.com/outchain/player?type=2&id=22707008&auto=1&height=32"></iframe>
</div>

<style>
    #music {
        position: fixed;
        right: 15px;
        bottom: 0;
        z-index: 998;
    }
</style>
    
    

  </div>
</body>

</html>